136 THE AMEBIC AN MONTHLY [June, 



Every drugg-ist knows that the Digitalis ptirptirca oi our gardens 

 contains a much less proportion of digitaline than that gi'own in the 

 wild state. Is it not possible that the specific bacillus of a given disease 

 may, under conditions not yet understood, likewise vary its product of 

 leucomaine? Or, to extend still further the shaky platform of possi- 

 bility, may we not in time rear a digitalis that will not contain digita- 

 line, and a Bacillus anthracis that will not in its life-history evolve 

 anthracine? 



It seems improbable that a jilant consisting of a single cell is less sus- 

 ceptible to variation than is an elaborate organism like Indian corn ; 

 vet the gross ditlerentiaj between the Indian corn grown in New England 

 and that grown in Kansas are greater in degree than the difterences 

 that characterize species in many genera — the genus Carex, for ex- 

 ample. 



Under changed environment, the wild boar, whicli produced only 

 brawn and bristles, has been transformed into the domestic hog, which 

 produces only fat. It is hard to believe that a microbe would not. under 

 equal change of conditions, show equal variation in form or product. 



An Interesting Experiment with the Microscope. 



By Prof. W. F. DlfRAND, 



AGRICULTURAL COLLEf.E, MICH. 



The following experiment, though not, perhaps, pertaining to micros- 

 copy in the strictest sense of the term, is, however, so beautiful and so 

 simple that no one having a microscope should fail of trying it. 



Before describing the nature of the experiment, it may be well to 

 .refer briefly to the structuie of the retina of the eye. This, as is well 

 known, consists in general of an expansion of the optic nerve over the 

 back of the eye between the choroid coat and the vitreous humor. Its 

 structure is verv complex, being divided by difterent writers into from 

 four to ten separate layers. On the anterior or inner surface, a little to 

 the outer side of the centre, is a yellow patch, within which is a well 

 marked depression. This yellow patch is called the macula httea^ and 

 the depression the fovea centralis. The macula Intea is the most 

 sensitive portion of the retina, and the fovea centralis is the part 

 whereon rests the image of an object directly looked at. That is, if 

 tlie eye directly looks at an ol)ject. its image is formed on this depres- 

 sion of the macula lutea. It is, therefore, that part of the retina 

 which does, so to speak, nearly all the work. The fibres of the mac?tla 

 hitea luive the direction of rays diverging in all directions from the 

 centre of \\-\& fovea centralis. One of the layers of the retina is that 

 of the rods and cones, as they aie called. The rods are long cylindri- 

 cal bodies packed in closely with their axes at right angles to the plane 

 of the retina. The cones are somewhat conical or bottle-shaped bodies, 

 interspersed among the rods. \\\ i\\<i fovea cc7itralis Xh^ cones alone 

 are found. These bodies — the rods and cones — are supposed to receive 

 the vibratory influence of the rays of light. This is then transmitted 

 to nerve cells connected with the filamentary expansion of the optic 

 nerve. 



Ramifying through all parts of the retina except the fovea centralis 

 is a capillary network of veins and arteries. This network, together 



